In electrophotography, a latent image is created on the surface of an imaging member which is a photoconducting material by first uniformly charging the surface and selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner.
Typically, a dual layer electrophotographic photoconductor comprises a substrate such as a metal ground plane member on which a charge generation layer (CGL) and a charge transport layer (CTL) are coated. The charge transport layer contains a charge transport material which comprises a hole transport material or an electron transport material. For simplicity, the following discussions herein are directed to the use of a charge transport layer which comprises a hole transport material as the charge transport compound. One skilled in the art will appreciate that if the charge transport layer contains an electron transport material rather than a hole transport material, the charge placed on the photoconductor surface will be opposite that described herein.
When the charge transport layer containing a hole transport material is formed on the charge generation layer, a negative charge is typically placed on the photoconductor surface. Conversely, when the charge generation layer is formed on the charge transport layer, a positive charge is typically placed on the photoconductor surface. Conventionally, the charge generation layer comprises the charge generation compound or molecule alone and/or in combination with a binder. The charge transport layer typically comprises a polymeric binder containing the charge transport compound or molecule. The charge generation compounds within the charge generation layer are sensitive to image-forming radiation and photogenerate electron hole pairs therein as a result of absorbing such radiation. The charge transport layer is usually non-absorbent of the image-forming radiation and the charge transport compounds serve to transport holes to the surface of a negatively charged photoconductor. Photoconductors of this type are disclosed in the Adley et al U.S. Pat. No. 5,130,215 and the Balthis et al U.S. Pat. No. 5,545,499.
The Champ et al U.S. Pat. No. 5,130,217 discloses dual layer photoconductors wherein the charge generation layer comprises solution squarylium formulations. Champ et al found that high molecular weight polymers containing carbonyl or sulfonyl groups degrade in the presence of amine solvents. Arylsulfonamide resins have been used to form solution squarlyium formulations, but the formulations typically must be used within a short time, for example thirty minutes, to ensure good coating quality and spectral properties. Other high molecular weight polymers which have been found to be stable in basic amine solvents were found to exhibit poor coating quality and/or unacceptable electrical photographic properties, e.g., high residual voltage and dark decay.
Squarylium dyes are thought to possess a preferred aggregation or intermolecular stacking which optimizes electronic transitions between molecules. As disclosed in Champ et al, binder systems which do not interfere with this stacking arrangement are preferred. The methyl bisphenol A (60% weight)/bisphenol A (40% weight) (MeBPA/BPA respectively) system disclosed by Champ et al allows aggregate formation because the polymers comprise small molecules. A mixture of these binders is used to minimize crystallization.
The squarylium-containing charge generation layers as disclosed by Champ et al are advantageous in that they provide good electrophotographic properties to dual layer photoconductors. In order to provide such photoconductors with improved durability, it would be advantageous to improve their abrasion resistance, and particularly resistance to layer delamination, while maintaining good electrophotographic properties.